I. Field of the Invention
The present invention relates to the inspection of semiconductor chips and in particular to the inspection of leads on a semiconductor chip using zero force part placement.
II. Discussion of the Prior Art
Inspection of leads on semiconductor chips requires accurate measurement of the seating plane of the part being inspected. However, since lead lengths are unknown prior to inspection, prior art methods have avoided placing a part on its leads because such placement may crush or deform the leads. Instead, a part for inspection is held with its leads suspended. As a result, the actual seating plane of the part, as determined by the length of its leads seated on a planar surface, is not measured directly during the inspection process.
Instead of placing the part under inspection onto its actual seating plane with its leads contacting a flat surface the prior art indirectly calculates the seating plane for the part. The calculation is made with the part's leads suspended in air. Measurements for the calculated seating plane are made by sighting along a line determined by, for example, the three longest leads on the part being inspected. In order for such an approach to be valid an assumption is made that the three longest leads encompass the center of gravity of the part being inspected. However, in many cases the three longest leads may not encompass the center of gravity of the part being inspected. Such a part rocks on its leads. The inspection technique of the prior art may result in a rocking part erroneously passing inspection. A rocking part is a part which will not seat correctly but instead exhibits a rocking motion when placed on a flat surface.
Further, prior art measurement systems are subject to measurement variability leading to relatively large errors for some lead measurements when making a measurement of coplanarity. The measurement of coplanarity is defined as the distance from the seating plane, whether calculated or actual, to the bottom of the lead tip. FIG. 1 shows one example of the potential errors introduced by prior art systems in measuring the seating plane of the semiconductor chip 10. In FIG. 1, the actual seating plane is denoted by line 18 which is a line through the center points of the lead tips of leads 12A and 12B. However, in calculating or measuring the coplanarity of the leads using prior art techniques, calculated seating planes are subject to uncertainties due to error factors inherent in the prior art measuring instruments. Further, because of such inherent error factors, results are not repeatable using prior art techniques.
Therefore, in one measurement, the prior art may measure leg 12A with error E1 resulting in an inaccurately long measurement of the lead which results in placement of the seating plane through points S1 and S2. This, in turn, results in the seating plane denoted by broken line 14 which places lead 12C directly on the calculated seating plane 14. Lead 12C is elevated significantly higher than the actual seating plane 18. Calculated seating plane 14 also results in an exaggeratedly large error for lead 12D which appears to be raised high above the calculated seating plane 14 when in fact, at the point of measuring, lead 12D is actually significantly closer to actual seating plane 18.
Measurement errors of as small as 0.001 inch along a length of 1 inch of the semiconductor chip are extremely significant in the art. An error factor of 0.0002 inches in measuring a given lead can easily lead to significant errors in the calculation of the seating plane. Such error factors are inherent in prior art apparatus.
Similarly, a second measurement taken along the broken line 16 introduces error E2 which finds coplanarity through points S3 and S2. As in the foregoing example, it similarly places legs 12C and 12D incorrectly with respect to the actual seating plane 18. Such measurement errors can easily lead to the acceptance of parts which are subject to rocking motion when placed on, for example, a substrate or printed circuit board.
It is therefore the motivation of the present invention to avoid the errors inherent in the prior art and to provide a method of locating the seating plane exactly and directly through the use of mounting the part by its leads onto a known reference plane.
It is a further motivation of the present invention to determine the thickness of a part where the part thickness is defined as the distance from the seating plane determined by its leads in the seated position to the top of the part.